In recent years, portable radio apparatuses have been reduced in size and weight. Accordingly, antenna assemblies used for such portable radio apparatuses are also required to be smaller. As an antenna assembly which satisfies the above requirements, many manufacturers are developing whip antennas which can be retracted into the housing body when it is used for communications. In earlier stage, portable radio apparatuses of relatively simple structure have utilized this type of whip antenna.
However, such a simple antenna has the following problem.
Specifically, the antenna, when extended from the housing body, is operative as a monopole antenna, whereas it cannot obtain a sufficient gain when retracted in the housing body. It can be considered that this is because the antenna retracted in the housing body is placed near the ground to cause an input impedance to increase, whereby impedance matching can not be established.
Thus, to improve the gain of the antenna when retracted in the housing body, a whip antenna of a so-called top loading type came into use instead of the simple whip antenna described above. The top loading type whip antenna refers to an antenna assembly which has a helical antenna electrically connected to the top end of a rod antenna.
When this type of whip antenna is extended from the housing body for use, radio waves can be radiated from both of the helical antenna and the rod antenna. When the antenna is retracted into the housing body, radio waves can be radiated from the helical antenna.
However, this type of whip antenna includes the rod antenna which does not contribute to the radiation of radio waves when it is retracted. The helical portion operates as an open stub.
The open stub may badly affect the input impedance of the antenna. Specifically, the open stub causes the impedance matching to be disrupted due to the distance between the rod antenna and a circuit board in the housing space and so on.
Thus, it cannot be said that this type of whip antenna is well designed. In addition, the whip antenna of top loading type, if not shielded completely, also has several problems, for example, that signals interfere from the retracted rod antenna and signals go into a shielded portion, and so on.
As a result of investigations for solving the above-mentioned problems, an antenna assembly has been developed, in which a rod antenna, when retracted into the housing body, is electrically isolated from a helical antenna. A prior art example of such an antenna assembly will be described with reference to FIGS. 1 to 6.
FIGS. 1 to 6 show two states of the antenna assembly equipped in a portable radio apparatus, that is, an extended state and a retracted state. It should be noted that in the drawings, illustration of the whole portable radio apparatus is omitted, and the antenna assembly and portions of the portable radio apparatus associated with the connection to the antenna assembly only are described. Also, corresponding parts in the various drawings are designated the same reference numerals.
First, the structure of an antenna assembly 1 shown in FIGS. 1 and 2 will be described. The antenna assembly 1 is composed of two antenna portions, that is, a rod antenna 1A and a helical antenna 1B. The rod antenna 1A is operative mainly when the antenna assembly 1 is extended from a housing body 2, while the helical antenna 1B is operative when the antenna assembly 1 is retracted into the housing body 2.
The antenna assembly 1 is attached to the housing body 2 by screwing an antenna connection fitting 1C on the antenna side into an antenna fixture 2A arranged in a non-metallic housing body 2.
The housing body 2 contains a circuit board 3 on which a variety of circuits as well as a power supply circuit 3A for the antenna assembly 1 are integrated. The power supply circuit 3A not only supplies the antenna assembly 1 with electric power through a power supply spring 3B but also has a function of matching a characteristic impedance of a transmitter/receiver circuit with an input impedance of the antenna assembly 1.
The power supply circuit 3A is, as shown in FIG. 1, electrically connected to the antenna assembly 1 when the antenna assembly 1 is extended from the housing body 2 and an extended-state limiter 1D arranged around a lower end portion of the rod antenna 1A is mechanically coupled to the antenna connection fitting 1C. Specifically, the power supply circuit 3A is electrically connected to the rod antenna 1A by way of the antenna fixture 2A, antenna connection fitting 1C, extended-state limiter 1D, and stopper 1E in that order.
In this way, the rod antenna 1A is brought into an operable state as a monopole antenna which has its ground level at the ground of the circuit board 3 and a shielding case.
Incidentally, the stopper 1E is provided for preventing the antenna assembly 1 from falling out. The extended-state limiter 1D and the stopper 1E are both made of a single metal part. The rod antenna 1A is caulked with the stopper 1E for establishing electrical and mechanical connection therebetween. Also, the rod antenna 1A is entirely covered with an antenna cover 1F or the like so as to prevent a human body from directly touching the rod antenna 1A.
On the other hand, when the antenna assembly 1 is retracted into the housing body 2 as shown in FIG. 2, the antenna connection fitting 1C is mechanically and electrically connected to a retracted-state limiter 1G, so that the helical antenna 1B can be powered from the power supply circuit 3A. In this state, the helical antenna 1B is operative as a helical antenna which has its ground level at the ground of the circuit board 3 and the shielding case.
The helical antenna 1B is also entirely covered with an antenna cover 1H so as to prevent a human body from directly touching it. The antenna cover 1H is utilized as a handle with which the user extends the antenna assembly 1, and also functions as a stopper for preventing the antenna assembly 1 from falling into the housing body of the radio apparatus when it is retracted.
As described above, since different antenna portions 1A, 1B individually operate when the antenna assembly 1 is extended and when it is retracted, favorable characteristics can be provided using the same matching circuit if impedances of the respective antenna portions are optimized.
An antenna assembly 4 shown in FIGS. 3 and 4 is also known as an antenna assembly which provides favorable characteristics as described above. Although the antenna assembly 4 is also composed of a rod antenna 4A and a helical antenna 4B, it differs from the antenna assembly 1 in the way each antenna is powered. More specifically, the helical antenna 4B is always powered from a power supply circuit 3A, while the rod antenna 4A is powered only when the antenna assembly 4 is extended.
When the antenna assembly 4 is extended from a housing body 2, the rod antenna 4A extends through the inside of the helical antenna 4B, as shown in FIG. 3. This causes the rod antenna 4A to electromagnetically couple to the helical antenna 4B, whereby the rod antenna 4A can be powered. In this state, the rod antenna 4A is operative as a monopole antenna which has its ground level at the ground of a circuit board 3 and a shielding case.
The rod antenna 4A is fixed to an antenna connection fitting 4C by an extended-state limiter 4E disposed near the lower end thereof when the antenna assembly 4 is extended. The extended-state limiter 4E is mechanically engaged with a non-metallic extended-state fixture 4D arranged in the antenna connection fitting 4C.
The rod antenna 4A and the helical antenna 4B of this example are also covered with antenna covers 4G and 4H, respectively, so as to prevent a human body from directly touching them. A non-metallic stopper 4F is provided for preventing the antenna assembly 4 from falling out.
FIG. 4 shows connections at various portions of the antenna assembly 4 when it is retracted in the housing body 2. When the antenna assembly 4 is retracted into the housing body 2, the antenna assembly 4 is fixed by a retracted-state limiter 4J formed near the top end of the antenna assembly 4 which is mechanically engaged with a retracted-state fixture 4I arranged in the antenna cover 4G. In this state, since the distance G from a handle 4K to the upper end of the rod antenna 4A is larger than the length HL of the helical antenna 4B, the upper end of the rod antenna 4A is located below an antenna fixture 2A, so that the rod antenna 4A is electrically isolated from the helical antenna 4B. Therefore, the helical antenna 4B only is operative in the retracted state of the antenna assembly 4.
The helical antenna 4B is operative as a helical antenna which has its ground level at the ground of the circuit board 3 and the shielding case. The handle 4K also serves as a stopper for preventing the antenna assembly 4 from falling into the housing body 2. The antenna assembly 4 thus constructed also provides favorable characteristics when it is extended as well as when it is retracted.
An antenna assembly 5 shown in FIGS. 5 and 6 is also known as an antenna assembly which provides favorable characteristics, similarly to the above-mentioned ones. The antenna assembly 5 has substantially the same structure as the antenna assembly shown in FIGS. 3 and 4. The feature of the antenna assembly 5 lies in that a rod antenna 5A operable when the antenna assembly 5 is extended is not powered through electromagnetic coupling but through mechanical and electrical connection.
More specifically, when the antenna assembly 5 is extended from a housing body 2, the rod antenna is fixed by mechanical and electrical connection of an extended-state limiter 5B arranged near the lower end of the rod antenna 4A to an antenna connection fitting 5A. The rest of the structure is similar to that of the antenna assembly 4. In addition, a stopper 5C is also provided for preventing the antenna assembly 5 from falling out.
After the antenna assembly 5 is extended from the housing body 2, the rod antenna 4A is mainly operative as an antenna which has its ground level at the ground of a circuit board 3 and a shielding case. Although a helical antenna 4B is also powered in this state, it is operative as a supplement to the rod antenna 4A.
On the other hand, when the rod antenna 4A is retracted into the housing body 2, the rod antenna 4A is electrically isolated from the helical antenna 4B since the distance G from a handle 4K to the upper end of the rod antenna 4A is larger than the length HL of the helical antenna 4B. As a result, the helical antenna 4B only is operable in the retracted state. In this way, the antenna assembly 5 favorably operates when it is extended as well as when it is retracted.
However, the above-mentioned three kinds of antenna assemblies 1, 4, 5 present a problem associated with a reduction in size of the portable radio apparatus. The problem is caused by the fact that the antenna length L must be shorter than the length of the housing body 2 for housing the rod antenna 1B or 4A in the housing body 2. However, since the antenna length L is essentially determined from wavelengths of used frequencies, it cannot be made shorter in order to reduce the size of the housing body 2. Generally, the antenna length L should be approximately one quarter to one half of wavelengths of communication frequencies.
Therefore, if a portable radio apparatus is to be made smaller, an antenna assembly designed according to the conventional scheme may not be completely housed within the housing body of the radio apparatus. For example, when the antenna assembly shown in the prior art example is used in a portable radio apparatus for a frequency band of 800 MHz!, the antenna length L must be approximately 90 mm! even if it is one-quarter wavelength. Thus, the antenna assembly cannot be used in a portable radio apparatus with a housing body, the size of which is 90 mm! or less.
Also, with the antenna assembly 5 having the structure shown in FIGS. 5 and 6, when the rod antenna 4A is extended, the rod antenna 4A and the helical antenna 4B are always connected simultaneously to the antenna fitting 5A, so that the rod antenna 4A is not operative as a simple monopole antenna. This constitutes the cause of lower design freedom.